Undesirable airborne compounds, including sulfur compounds, ammonia, formaldehyde, urea, carbon monoxide, oxides of nitrogen, mercaptans, amines, and ethylene, occur in a number of environments, where most are primarily responsible for the presence of disagreeable odors, or irritating or toxic gases. Such environments include petroleum treatment and storage areas, sewage treatment facilities, hospitals, morgues, anatomy laboratories, animal rooms, and pulp and paper production sites, among others. These undesirable compounds may be bacterial breakdown products of higher organic compounds, or byproducts of industrial processes.
Hydrogen sulfide H2S, a colorless, toxic gas with a characteristic odor of rotten eggs, is produced in coal pits, gas wells, sulfur springs, and from decaying organic matter containing sulfur. Controlling emissions of this gas, particularly from municipal sewage treatment plants, has long been considered desirable. More recently, protecting electronic apparatus from the corrosive fumes of these compounds has become increasingly important. Further, H2S is flammable.
Ammonia (NH3), also a colorless gas, possesses a distinctive, pungent odor and is a corrosive, alkaline gas. The gas is produced in animal rooms and nurseries and its control also has long been considered desirable.
Chlorine (Cl2) is a greenish-yellow gas with a suffocating odor. The compound is used for bleaching fabrics, purifying water, treating iron, and other uses. Control of this powerful irritant is most desirable for the well being of those who work with it or are otherwise exposed to it. At lower levels, in combination with moisture, chlorine has a corrosive effect on electronic circuitry, stainless steel and the like.
Formaldehyde (HCHO) is a colorless gas with a pungent suffocating odor. It is present in morgues and anatomy laboratories, and because it is intensely irritating to mucous membranes, its control is desirable.
Urea (CH4N2O) is present in toilet exhaust and is used extensively in the paper industry to soften cellulose. Its odor makes control of this compound desirable.
Carbon monoxide (CO), an odorless, colorless, toxic gas, is present in compressed breathing air. Oxygenation requirements for certain atmospheres, including those inhabited by humans, mandate its control.
Oxides of nitrogen, including nitrogen dioxide (NO2), nitric oxide (NO), and nitrous oxide (N2O), are compounds with differing characteristics and levels of danger to humans, with nitrous oxide being the least irritating oxide. Nitrogen dioxide, however, is a deadly poison. Control of pollution resulting from any of these oxides is desirable or necessary, depending on the oxide.
Mercaptans and amines, including methyl mercaptan (CH3SH), butyl mercaptan (C4H9SH) and methyl amine (CH5N), are undesirable gases present in sewerage odor. The control of these gases is desired for odor control.
Ethylene (C2H4) is a colorless, flammable gas that is a simple asphyxiant which accelerates the maturation or decomposition of fruits, vegetables, and flowers. Control of this compound prolongs the marketable life of such items.
Attempts have been made to provide solid filtration media for removing the undesirable compounds listed above from fluid streams. Desired features of such media are a high total capacity for the removal of the targeted compound, a high efficiency in removing the compound from an air stream contacting the media, and a high ignition temperature (non-flammability).
One specific example of a solid filtration media for the removal of undesirable compounds from gas streams is described in U.S. Pat. No. 4,235,750. The '750 patent discloses an apparatus and method for absorbing ethylene and other gaseous contaminants, wherein the apparatus is a three-part container comprising permanganate impregnated alumina in one compartment, activated carbon in the second compartment, and a mixture of molecular sieves and activated silica gel in the third compartment.
Although the '750 patent discloses a potassium permanganate impregnated alumina for the removal of undesirable compounds from fluid streams, the capacity of the impregnated alumina is limited. The efficiency of the permanganate impregnated alumina of the '750 patent is limited as its optimal concentration of permanganate is 4.5%, and higher concentrations of permanganate results in the clogging of the pores of the substrate and therefore its oxidizing capacity being reduced. Accordingly, this filtration media would be limited to approximately a 9% capacity for the uptake of hydrogen sulfide gas in a gas stream. Therefore, this filtration media could not be efficiently used in small filter beds as larger quantities of the impregnated alumina must be used to compensate for its limited capacity. Further, the use of the impregnated alumina of the '750 patent would be more costly as the media would have to be replaced more frequently, thereby incurring the cost of more frequently purchasing the media and also incurring the cost of the additional labor required for its more frequent replacement. Finally, the permanganate impregnated alumina of the '750 patent is limited in that the failures in the adsorption of contaminants in fluid streams which occur at the end of the useful life of the media would be more frequent due to the limited capacity of the media. Therefore, the media of the '750 patent could not practically be utilized in systems where the air quality is critical.
There are other problems associated with the above described products. One is that they are dusty and may cause irritation to skin, eyes and the respiratory tract. Another problem is that the materials cannot be reused, i.e. they cannot be regenerated. Whatever cannot be saved for recovery or recycling should be handled as hazardous waste. Potassium permanganate in this form is incompatible with organic materials, combustible materials, strong reducing agents, strong acids, peroxides, chemically active metals and may also be toxic to aquatic life.
Another example of a solid oxidizing system in pellet to form consisting of activated alumina (Al2O3) impregnated with potassium permanganate (KMnO4) is described in U.S. Pat. No. 3,049,399. The pellets disclosed in the '399 patent provide air purification and odor control by both adsorbing and absorbing odors, and then destroy the collected odors by the potassium permanganate's controlled oxidizing action.
The potassium permanganate impregnated alumina pellets of the '399 patent are limited in that they have a limited capacity for removing undesired contaminants from gas streams.
Yet another example of a solid filtration media for removing undesirable compounds from a gas stream is disclosed in U.S. Pat. No. 3,226,332. The '332 patent teaches a method of producing granular activated alumina uniformly impregnated with a solid oxidizing agent, preferably potassium permanganate, for use in treating fluid streams. This method includes the spray addition of the impregnate, wherein the impregnate solution is sprayed onto the dry combination being tumbled in a mixer thereby forming pellets which are later dried to remove a substantial portion of the remaining water.
U.S. Pat. No. 6,004,522 (Purafil) addresses the problems illustrated above, i.e. the need in the prior art by providing a high efficiency, high capacity, low flammability permanganate impregnated substrate for the removal of undesirable contaminants from gas streams. There is disclosed a long lasting filtration media which needs to be replaced less frequently and therefore minimizes maintenance and replacement costs. Also provided by the impregnated substrate of the present invention is a high capacity filtration medium, which may be used in small filter beds, and therefore may allow the treatment of fluid streams where there are significant space limitations.
The filtration media of '522 is claimed to have a higher efficiency and capacity to remove certain undesired compounds from gaseous streams than do the media in the prior art.
However, it still suffers from the disadvantage of using a medium comprising alumina impregnated with potassium permanganate, which is a compound that would be desirable to eliminate, in that it is associated with environmental problems, as indicated above.
It is also desirable to extend the useful life of a filter medium further, beyond the limits of the above prior art media.
Another type of material usable for filtering purposes is so called aerogels. This material has remarkable properties, in terms of extremely low density, high porosity, good thermal insulation capacity. It has been employed in tests for filtering purposes. However, its manufacture in large scale is expensive, and thus it offers no economically feasible alternative to the above discussed media.
In a prior art sol-gel technique, commonly used to produce homogeneous gels and powders with high surface area, e.g. aerogels, an intermediate product is a gel. Gelling involves particles, which are linked together in branched chains that fill the whole volume of the solution so that there is no increase in the concentration of silica in any macroscopic region of the medium, i.e. the silica is uniformly (on a macroscopic level) distributed in the media. Thus, the overall medium becomes viscous and will subsequently be solidified, forming a coherent network of particles, which retains the liquid by capillary action.
EP 0 476 135 A1 discloses an adsorbent which is ammonium ion and ammonia selective and a process for making such adsorbents. The process involves a dropwise addition of water to an aluminium salt or an aluminate and a silicate in alkaline conditions. The reaction mixture is heated and then the pH is adjusted to a pH of 4 to 9 to bring about a precipitation. The product is aged and the product is dried an used as an agent for the adsorption of ammonium ions. This publication does not disclose anything but adsorption of ammonium ions and ammonia. In particular it does not relate to absorption of any other gaseous species than ammonia.
U.S. Pat. No. 3,755,183 discloses a process for making silicate adsorbents and crying agents by forming precipitation products from alkali metal salt solutions of salts containing di- and trivalent metals. The product is based on the use of one metal ion, and the use of the product is for adsorbing oils and moisture.